Rob Jackson, Marielle Saunois, Philippe Bousquet, Pep Canadell & Ben Poulter, SCIENTIFIC AMERICAN
Atmospheric concentrations of the second most important greenhouse gas are hitting record levels
Cows, oil and gas wells, rice paddies, landfills. These are some of the biggest sources of methane staining the atmosphere today. Methane is the most important greenhouse gas after carbon dioxide, and its concentration reached a record 1,875 parts per billion (ppb) last year, more than two and a half times preindustrial levels. Peak methane in the atmosphere feels as elusive as a cure for the (next) coronavirus.
As scientists at the Global Carbon Project, we and dozens of our colleagues just published a four-year study and public data sets of the Global Methane Budget to estimate methane sources from land, oceans, agriculture and fossil fuel use. Methane emissions reached a record 596 million metric tons per year (range of 572–614 million tons including error estimates) in 2017, the last year for which data are fully available. We present the results in the journals Earth System Science Data and Environmental Research Letters.
More than half of global methane emissions come from human activities, primarily agriculture and fossil fuel use. Our estimate for 2017 is up about 50 million tons, or 9 percent, compared to annual methane emissions in the early 2000s. Convert those 50 million extra tons of methane each year to the warming potential of carbon dioxide over the next century, and we’ve added the equivalent of 350 million more cars to the world’s roads—or another Germany and France to the world’s emitters.
The concentration of methane in the atmosphere is tracking trajectories modeled in aggressive warming scenarios where global temperatures rise by three to four degrees Celsius this century. With each passing year, we move further away from the pathways that climate models suggest will hold warming below 1.5 or two degrees C. In many ways we’re even further from reducing methane emissions than we are for carbon dioxide.
Biological sources of methane arise primarily from microbes growing in low-oxygen environments, including natural wetlands, landfills, water-logged rice paddies and the stomachs of ruminant cows, goats and sheep. We don’t find evidence for increased methane emissions from natural wetlands, but we do from landfills and ruminants through 2017; there are a billion and a half more people on earth today than in the year 2000, and average red meat consumption per person is still increasing. Agriculture contributes about two thirds of all methane released from human actions—as much as all natural sources combined.
Natural seeps, such as bubbling mud volcanoes, release some methane from fossil sources underground. But most fossil geologic methane making its way to the atmosphere comes from fossil fuels we extract, transport or burn. After agriculture’s two-thirds contribution, fossil fuel activities contribute most of the remaining third of global methane emissions from human actions—from coal mines and oil and gas wells to leaky natural gas pipelines and kitchen stoves. Overall, emissions from agriculture and fossil fuel use contributed equally to the 50-million-ton annual increase we observed.
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